Fire pumps are normally driven by engines of the internal combustion type. If the throttle controlling the engine driving the pump remains at one setting, the pump discharge pressure fluctuates with the load imposed on the engine by the pump. With a number of hose lines connected to the pump, if one hose line is closed thus reducing the load, the pressure in the remaining hose line increases, creating a definite hazard to those handling the remaining lines. Similarly, when an added hose line is turned on, the remaining hose lines experience a drop of pressure complicating the control thereof. Thus, particularly in combination with fire pumps, it is very important to maintain as nearly constant a discharge pressure as is possible.
Devices have been designed to attempt to maintain a relatively constant discharge pressure by controlling the speed of the engine driving the pump. See, for example, U.S. Pat. No. 2,642,805 to Richard Curt Nepper, granted June 23, 1953; U.S. Pat. No. 3,116,694 to Harry A. Eberhardt, granted Jan. 7, 1964; and U.S. Pat. No. 3,544,235 to Edson W. Smith, granted Dec. 1, 1970.
Attempts have been made to control the discharge pressure of fire pumps through the use of a piston and cylinder arrangement in which the piston is connected to the engine throttle or speed control device. One side of the piston in the cylinder is subjected to pump discharge pressure. The end of the cylinder on the other side of the piston is subjected to a predetermined reference pressure. When the pump discharge pressure increases, the piston moves toward the end of the cylinder subjected to the reference pressure, the piston rod moving the engine throttle toward engine idling speed, or reducing the speed of the engine to correspondingly reduce the discharge pressure. If the discharge pressure of the pump decreases, due to the opening of additional hose lines or for any other reason, the reference pressure exceeds the discharge pressure, and moves the piston and piston rod to open the throttle to increase the speed of the engine. See the patent to Smith, U.S. Pat. No. 3,544,235.
In the Smith device, and in other devices, the reference pressure is maintained through the use of an accumulator containing air which is compressed by water to the extent necessary to equal the desired discharge pressure of the pump. Thus the pressure on opposite sides of the piston is normally equal. In view of the fact that the reference pressure remains substantially constant, the movement of the piston either in one direction or the other is controlled by variations in the pump discharge pressure.
The controls which have been previously produced include structures in which the cylinder is held in fixed position, and the piston may move relative to the cylinder to provide the control. See all three of the aforementioned patents. Where the piston sticks in one position, the throttle cannot be opened or closed until the piston is broken loose. This presents a dangerous and very unsatisfactory situation where the apparatus is being used to supply water under pressure during a fire. Such a condition is difficult to override or correct through the use of manual controls, and if shock on the control cylinder does not loosen the piston, shutdown of the equipment, and consequent loss of pressure in the hose lines, may be necessary.